1. Field of the Invention
The present invention relates to internal combustion engines. More particularly, the present invention relates to a stabilized bearing structure for rotatably supporting a crankshaft in a crankcase of an internal combustion engine, and to an engine incorporating the stabilized bearing structure.
2. Description of the Background Art
Many different types of internal combustion engines are known and commercially
Generally, in a single-cylinder internal combustion engine for a motorcycle or the like, a crankshaft is rotatably supported on a crankcase through a left-right pair of rotary bearings, a piston slidable in a cylinder bore is connected to a crank pin of the crankshaft through a connecting rod. During engine operation, an explosion pressure exerted on the piston is transmitted through the connecting rod to the crankshaft, and from the crankshaft to the crankcase through the rotary bearings.
In this type of engine, it is desirable that the inner and outer races of the bearings are securely attached to their respective engine components. Many of the known prior art bearings, therefore, have been respectively press fitted over journal shaft portions of the crankshaft and inside bearing portions of the crankcase, respectively. As a result, a rattling sound, which might otherwise occur due to backlashes between the fitted components, is suppressed, and a desired bearing function for the crankshaft is maintained, ensuring smooth and light rotation.
With such a known configuration, however, if the races of the rotary bearings are respectively press fitted over the crankshaft and into the crankcase, it then becomes a difficult to mount the crankshaft in the crankcase, and to subsequently perform maintenance thereof.
In order to solve this problem, conventionally, an engine structure and method of assembly has been adopted, in which the outer race of the right bearing is press fitted into a right crankcase half, and the inner race of the left bearing is press fitted over a left-side journal shaft portion of the crankshaft, prior to mounting the crankshaft onto the crankcase. This conventional engine structure is shown in FIG. 9.
At the time of mounting the crankshaft into the crankcase, a right-side journal shaft portion of the crankshaft is press fitted in the inner race of the right bearing (of which the outer race is already press fitted in the right-side crankcase half). Then, the outer race of the left bearing, with the inner race thereof already press fitted over the left-side journal portion of the crankshaft, is play fitted (fitted, with a play) into a left-side crankcase half, thereby facilitating the mounting of the crankshaft onto the crankcase and the maintenance thereof.
However, in this type of conventional structure, some play remains between the left outer bearing race and the left crankcase half after assembly. As a result, during operation of the engine, a little “backlash” is unavoidably generated between the outer race of the left bearing and a bearing surface of the crankcase. Therefore, a rattling sound due to the “backlash” is generated upon the operation of the internal combustion engine, and the desired bearing function cannot be optimized. structure in which the outside surface of the outer race of either one (preferably, the left-side one) of a pair of ball bearings is pushed in the axial direction by a push plug having an inclined push surface, so as to absorb the “backlash”. In the structure disclosed in this reference, however, the pushing force exerted on the outer race by the push plug is divided into a component force in the radial direction (for absorbing the “backlash”) and a component force in the axial direction, with the result that the component force in the axial direction acts as a force for moving the outer race in the axial direction.
Meanwhile, in the structure disclosed in Japanese Laid-Open Patent No. 2003-83080, the ball bearing (in which a resistance force for receiving the force in the axial direction is present) is adopted for each of the rotary bearings, for the crankshaft. The component force in the axial direction generated by the pushing force from the push plug can be satisfactorily received by the ball bearing (the right-side ball bearing) on the side of not being pushed by the push plug, and shifting of the bearing, due to the component force in the axial direction, can be avoided.
However, the ball bearing is bulkier in the radial direction than a roller bearing or a needle bearing, in comparison for the same rated capacity, leading to a larger occupying space of the crankshaft in the radial direction and, hence, to an increase in the overall size and weight of the internal combustion engine. Therefore, there is a need for an improved crankshaft support bearing system using at least one bearing other than the ball bearing, as at least one of the bearings for rotatably supporting the crankshaft.